control of respiration respiratory centre as an integrator of inputs from chemoreceptors, other...
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Control of Respiration
• Respiratory centre as an integrator of inputs from chemoreceptors, other receptors and higher centres
• Exercise
• Chemoreceptors: Peripheral (respond to changes in O2, CO2 and pH
• Inputs from other receptors
• Outputs to respiratory muscles and muscles of upper airway
Regulation of Ventilation
Inputs
• higher centres
• chemoreceptors
• “visual receptors”
Brain stem
Integrator
neural control
Respiratory centres
Outputs
• muscles of respiration rate & depth
• smooth mucle airways
• muscels upper airways (esp to during inspiration)
Higher centres:• voluntary control• speech• emotions: anxiety, shock• exercise
(joint position sense?)
Chemoreceptors
PCO2
pH
PO2
Peripheral:carotid bodies (aortic bodies)
respond to:
PaCO2
pH
PaO2 respond to:
PCO2
Central chemoreceptors:brain stem: near 3rd ventricle (cerebrospinal fluid)near respiratory centre
PaCO2 PcsfCO2
For PCO2, changes are sensed by:Peripheral
chemoreceptors20%
rapid response
Central chemoreceptors
80%somewhat slower
+
CO2 + H2O H2CO3 H+ + HCO3
actually sensed
PaCO2 ventilation
PaCO2 ventilation
Ve
nti
lati
on
(L
/ m
in)
PaCO2 (mmHg)
20 40 60
Ve
nti
lati
on
(L
/ m
in)
PCO2 (mmHg)20 40 60
C sensitivity
B sensitivity
Factors which affect slope of relationship:• gender, ethnic origin• sleep (slow wave sleep) — B• respiratory depressants — B
alcohol, barbiturate, anaesthetics, narcotics(unconsciousness)
• low PO2: hypoxia — C
Ventilatory Response to CO2
1. Response occurs at normal PaCO2
2. At very high PaCO2 (80 mmHg)
CO2 itself acts as respiratory depressant
3. Tolerance occurs
Cont...
Tolerance to PCO2:
• Most CO2 response due to central chemoreceptors within brain side of blood brain barrier close to cerebrospinal fluid
CO2 H2CO3 H+ + HCO3
• Local pH regulation
takes place over 13 days; cells lining 3rd ventricle can secrete HCO3
+
O2 response via carotid bodies (aortic body)
•small (2 mg) collections of neural tissue at
bifurcation of common carotid artery
• very high blood flow (equivalent of 2L/100g/min cf 54 ml/100g/min brain)
probably respond to dissolved O2
i.e. PaO2 not O2 content
response impaired in anemia, CO poisoning
• response present if blood flow or
blood pressure e.g. shock
• response caused also by cyanide
carotid body receptors activated by: nicotine
Response to hypoxia
1. Under normal circumstances
i.e. normal CO2
PO2 ventilation until PO2 falls to
60mmHg
2. A high PO2 does not inhibit ventilation
3. If PCO2 is high
that sensitivity to hypoxia
4. Tolerance does not occur
pH
• mainly sensed peripherally
• H+ doesn’t cross blood brain barrier well
• response to 7.3 – 7.5
pH ventilation
mild response cf PCO2
Visceral Receptors• Visceral reflexes that affect ventilation
cough, sneeze
vomit
• Stretch receptors in lung
Hering – Breuer reflex:
inflate lungs – stretch receptors detect stretch
respiratory centre to stop inspiration
Cough & Sneeze ReflexesAfferent sensory input Brainstem
medullaIrritation: Cough – sensory endings
in wall of extrapulmonary
respiratory tractsvagus
Irritation: Sneeze – sensory endings in nose & upper
pharnyxcranial nerveV
Deep inspiration followed by
Forced expiration against closed
glottis intrathoracic
pressureSudden glottic
opening
Forced expiration(nose)
Rapid expulsion air at high speed through mouth
(cough)
Clears irritant
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